The present invention relates to an insulated electric power cable.
Among plastic insulated electric power cables, a cross-linked polyethylene insulated electric power cable, having an insulating layer composed of a cross-linked polyethylene, has various advantages of not only excellent electrical property, mechanical property, and heat-resistance property, but also easy maintenance. Because of these advantages, the cross-linked polyethylene insulated electric power cable is the mainstream type of power-transmission cable. Generally, an insulator layer of the cross-linked polyethylene insulated electric power cable is formed by a method that comprises steps of; extrusion-coating, on a conductor, a cross-linkable resin composition, composed of a low density polyethylene blended with a cross-linking agent, an anti-aging agent, and the like, so as to cover the conductor therewith, and; then heating under pressure, so that the cross-linking agent can be thermally decomposed, to cross-link the resin composition. However, when a direct-current voltage is applied to the cross-linked polyethylene insulated electric power cable, a space electric charge is accumulated in the insulator layer, and a region of high electric field is formed locally. Consequentially, the problem arises that the breakdown voltage considerably decreases.
Several methods are proposed to solve this problem. For example, JP-A-62-100909 (xe2x80x9cJP-Axe2x80x9d means unexamined published Japanese patent application) discloses that a maleic anhydride-grafted polyolefin is blended with polyethylene. JP-A-61-253705 and JP-A-4-368717 each disclose that carbon black and/or magnesium oxide are added to a resin composition used for an insulator layer.
However, if a filler, such as carbon black or magnesium oxide, is added to the conventional electrically insulating resin composition, another problem arises: the impulse breakdown strength decreases. Modification of the resin composition by maleic anhydride does not arise a problem of the impulse breakdown strength, but the modification fails to give a satisfactory property to the direct-current breakdown strength.
According to the present invention there are provided the following means:
(1) An insulated electric power cable having an insulator layer formed by extrusion-coating and cross-linking a cross-linkable resin composition, which comprises a polyolefin blended with a total of 0.05 to 0.4 parts by mass of at least one compound selected from the group consisting of bis(3-ethyl-5-methyl-4-maleimidophenyl)methane, 2,2-bis[4-(4-maleimidophenoxy)phenyl]propane, N,Nxe2x80x2-m-phenylenebismaleimide, N,Nxe2x80x2-(4,4xe2x80x2-diphenylmethane) bismaleimide, and a compound represented by the following formula (1), based on 100 parts by mass of the polyolefin, and a prescribed amount of an organic peroxide cross-linking agent: 
wherein n is an integer of 1 or more.
(2) The insulated electric power cable described in the preceding item (1), wherein the organic peroxide cross-linking agent is a compound for which 1-hour half-life temperature is at least 140xc2x0 C.
(3) The insulated electric power cable described in the preceding item (1), or (2), wherein a prescribed amount of an anti-scorching agent is further added to the cross-linkable resin composition.
(4) The insulated electric power cable described in any of the preceding items (1) to (3), wherein the cable is a direct-current electric power cable.
Other and further features, and advantages of the invention will appear more fully from the following description.